1. Field of the Invention
This invention relates in general to optical waveguide suitable for use with optical second harmonic generating devices (hereinafter referred to "optical SHG devices") and to a method for manufacturing such devices.
2. Description of Related Art
Application of a LiNbO.sub.3 optical waveguide having a periodic inverted domain structure formed by periodically controlling its domains to optical SHG, for example, by quasi phase matching (abbreviated to "QPM") is described in E. J. Lim et al., Electric Letter. 25 (1989) 731 and J. Webjorn et al., IEEE Photon. Tech. Lett. 1 (1989) 316. Such a periodic inverted domain structure is formed by a method that diffuses Ti into a LiNbO.sub.3 substrate or a method that diffuses out Li.sub.2 O from a LiNbO.sub.3 substrate. Periodic inverted domain structures formed by those methods have a construction as shown in FIG. 4. As shown in FIG. 4, the periodic inverted domain structure has inverted domains 102 having a triangular section with depths of d, arranged with a period of .LAMBDA., and have a direction of spontaneous polarization deviating at an angle of 180.degree. from the direction of spontaneous polarization Ps of a substrate 101 of a z-plate of LiNbO.sub.3 single crystal (a plate of single crystal having major surfaces perpendicular to the c-axis of the crystal), and are formed by selectively diffusing Ti into the +c-surface of the substrate 101 or by selectively diffusing out Li.sub.20 from the +c-surface of the substrate 101. In this periodic inverted domain structure, d is approximately .LAMBDA./4.